The goal is to ascertain the differentiation steps in lymphocyte ontogeny in which terminal transferase synthesis is induced, and in which synthesis is terminated; and to further determine the differentiative potential of cells transformed by Abelson-Leukemia Virus and chemical carcinogens. Cells have been purified from murine thymus and bone marrow that can be induced to express Thy-1, terminal transferase, and the Thymus-Leukemia antigen (TL). T-cell hybrids of these sets have been formed by cell fusion with a HAT sensitive T-cell tumor line, and some of these are inducible by thymopoietin to express Thy-1 or TL at low levels. Initial studies indicate that the hybrids carry unrearranged C-mu genes from both the AKR tumor line used for fusion and the B6 cells isolated from the thymus. These tumor cell models will be used to study the cell biology of differentiation steps and the molecular biological consequences of those differentiation steps. cDNA clones are currently being made from our partially purified mRNA for terminal transferase. Expression vectors for human and murine cDNAs will be constructed, and used to produce antigen and hopefully functional enzyme. The 60,000 dalton entity coded for by this RNA will probably have new activities, not originally observed in the proteolytic product previously studied. Plans are being made to use the cDNA clones to clone the genes for the enzyme, from both human and murine genomic libraries.